Cementing plug launching mandrel

ABSTRACT

A cementing plug launching mandrel which includes an annular piston chamber and a piston in the chamber operable by fluid pressure in the mandrel to provide a force to shear the pins which support the cementing plug on the mandrel when a closure carried in the fluid closes the opening through the cementing plug. In addition, the mandrel has a surge chamber to absorb the shock pressure wave caused by closure of the opening through the cementing plug.

United States Patent Lyle B. Scott South Gate, Calif. 29,973

Apr. 20, 1970 Nov. 2, 1971 Byron Jackson, Inc. Long Beach, Calif.

inventor Appl. No. Filed Patented Assignee CEMENTING PLUG LAUNCHINGMANDREL 13 Claims, 6 Drawing Figs.

US. Cl 166/155, 166/5, 166/70 Int. Cl ..E2lb33/l6, E21b 33/14 Field ofSearch ..166/.5, 291, 70,153,155,156

[56] llle ferenc es Cited UNITED STATES PATENTS 2,630,179 3/1953 Brown166/155 X 2,704,980 3/1955 Vincent..... 166/153 X 3,076,509 2/1963 Burnset a1 166/153 X 3,364,996 l/l968 Brown 166/155 X 3,507,325 4/1970 Scott166/.5

Primary Examiner-David H. Brown Attorneys-Donald W. Banner, William S.McCurry and John W. Butcher PATENTEDNHV 2 ml 3.616.850

SHEEI 2 [IF 3 [NVENTOR CV66 5 SCOTT WA c9.

BACKGROUND OF THE INVENTION In the cementing of the well casing inoffshore wells drilled beneath a body of water, and wherein the casinghead apparatus is located on the floor of the ocean or other body ofwater, the cementing plug or plugs may be run into the well bore alongwith the casing string on a running in string of drill pipe. Suchpractice and apparatus for accomplishing such practice are moreparticularly disclosed in my pending application for U.S. Letters Pat.Ser. No. 735,685, filedJune 10, 1968, now US. Pat. No. 3,545,542. In theuse of suchapparatus, for example, a bottom cementing plug is launchedfrom its support in the casing when an opening through the bottom plugis closed off by a closure entrained in the cement slurry as it flowsdownwardly through the running in string-of drill pipe, throughthe'plug-launching mandrel, and through" the bottomcementing plug. Thecement slurry, being substantially heavier than the fluid preceding itin the well bore generally causes the preceding fluid or flushing fluidto be rapidly downwardly displaced in the casing and upwardly in theannulus between the casing and the well bore wall. When this occurs, thewell is said to be on vacuum" and the casing below the top cementingplug, which is still retained on its mandrel, as well as therunning-instring of drill pipe are devoid of hydraulic fluid or cement.Thereafter, a closure for the opening through the top cementing plug isintroduced into the running-in string of drill pipe in advance of aquantity of displacing fluid, which is to be employed to finallydisplace the cement slurry into the casing well bore annulus. It isdesirable that the separation of the top cementing plug from itsmand'rel be positively determinable, since generally in this type ofcementing operation, the volume of the displacing fluid above the topplug is calculated, based on the known volume of the casing string andthe volume of the casing well bore annulus. However, since the topcementing plug is located substantially below the top of the running-instring of drill pipe, where the well is being completed in water whichmay be hundreds of feet deep, the displacement fluid above the closurefor the top plug opening attains substantial velocity during itsdownward travel towards the top cementing plug. When the closure closesthe top plug, there is an instantaneous shock pressure wave or waterhammer effect which acts on the top plug, and in the case where themandrel has a port above the top plug to expose the entirecross-sectionalarea of the plug to displacing fluid pressure todischarge or launch the plug, the plug may be prematurely launched as aresult of the shock pressure wave. As a consequence, pressure cannot bedeveloped in the displacing fluid for the purpose of giving a sensiblepressure increase at the top of the running-in string of drill pipe, thedrop in which pressure would otherwise serve to signify that the topplug has been launched.

While the problem is more or less acute in plugs of different sizes,from large cementing plugs used to displace cement in the cementing ofthe usual conductor pipe in the well bore, downwardly to the smallerstring of casing which is ultimately run into the well and cemented inplace, the problem is basically caused by the fact that if the shearpinswhich connect the top plug to the mandrel are so strong as to preventpremature launching of the plug by the above-referred shock pressurewave acting across the entire cementing plug area, then a verysubstantial increase in displacing fluid pressure must be relied upon toeffectively displace the plug by shearing the pins, such high pressurepossibly resulting in the flow of fluid past the outside of the plugwithout causing shearing of the connection of the plug to the mandrel.On the other hand, while the problem can be somewhat reduced byconfining the displacing fluid to the running-in string of drill pipe,thereby reducing the effective area of the plug to which displacingfluid pressure is applied, such solution involves the objectionablereduction of the shear strength of the shear pins which connect the topplug to its mandrel, with resultant uncertainty as to the launching ofthe plug.

SUMMARY OF THE INVENTION The present invention provides the cementingplug launching apparatus which obviates the foregoing problems.

More particularly, the present apparatus includes a pluglaunchingmandrel adapted to be connected to the running-in string of drill pipefor the conduct of cement and other fluids into the casing below thecementing plug apparatus, and wherein the displacing fluid which is toshear the top plug from the mandrel is confined to the mandrel itself,and wherein the mandrel has an annular piston chamber provided with anannular piston responsive to the pressure of displacing fluid, when thepassage through the top cementing plug is closed by the usual closure ordart, to provide a supplemental force for shearing the shearpins whichconnect the plug to the mandrel.

In addition to the foregoing, the present apparatus also includes in theplug-launching mandrel a surgechamber which is evacuated when the cementslurry rapidly moves downwardly in the wall casing placing the wellheadon vacuum, and which enables the dampening of the shock pressure waveoccasioned by the closure or dart blocking the flow of displacing fluidthrough the top plug, so that the water hammer effectwill notinadvertently displace the top plug.

As a-result of the foregoing, applicant's invention provides a meanswhereby a'predetermined pressure must be applied to the annular'pistonas well as to the plug closure in order to displace the plug from itsmandrel, and the instantaneous drop in such pressure will suffice topositively signify at the top of the running-in string of drill pipethat the plug has been displaced. Thus, the invention affords greatercertainty for the well-cementingcrew.

This invention possesses many other advantages, and has other purposeswhich may be made more clearly apparent from a consideration of a formin which it may be embodied. This form is shown intlie drawingsaccompanying and forming part of the present specification. It will nowbe described in detail, for the purpose of illustrating the generalprinciples of the invention; but it is to be understood that suchdetailed description is not to be taken in a limiting sense, since thescope of th'einvention is best defined by the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a view partly in elevationand partly in vertical'section illustrating plug-launching apparatusmade in accordance with the invention installed in association withunderwater wellhead equipment apparatus;

FIG. 2 is a view in vertical section and on an enlarged scaleillustrating the plugs and mandrel in the wellhead body shown in thelower portion of FIG. 1;

FIG. 3' is a view in vertical section showing the bottom plug releasedand providing a barrier between flushing fluid below the bottom plug andcement slurry above the top plug, prior to the release of the top plug;

FIG. 4 is a view in vertical section showing the launching dart for thetop plug seated in the top plug, prior to release of the top plug;

FIG. 5 is a view in vertical section showing the release of the topplug; and

FIG. 6 is a view in vertical section showing the top plug nested in thebottom plug following displacement of the cement slurry from the casing.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. 1, thereis generally illustrated wellhead apparatus H of the type adapted to belocated beneath the water adjacent to the floor F or bottom of thewater. Cementing and other completion operations are performed throughthe wellhead apparatus from a vessel or platform at or above the surfaceof the water, and for cementing operations the surface apparatusincludes a cementing head CH. The well includes a large bore W in whichan outermost string of easing C is disposed and cemented into place. Theupper end of the casing C has a seat 1 for a supporting flange 2 of thetubular wellhead body 3 which supports another string of well casing Cladapted to extend downwardly into a reduced well bore section and to becemented in place. Internally thereof the wellhead body 3 may support acasing hanger body 4 for another casing string (not shown) adapted toextend still further into a further reduced well bore and to be cementedin place. Supported on the casing hanger 4 is an innermost casing hangerbody 5 connected as at 6 to an innermost casing string C2 which isillustrated as extending downwardly into the reduced well bore W2. Thecasing hanger body 5 and the casing string C2 are run into the well on auniversal running tool T which is threadedly connected as at 7 to thelower end of a string of drill pipe 8, the running tool T beingthreadedly connected as at 9 to the upper end of the casing hanger body5.

Such subsurface apparatus also generally includes an annular supportmember 10 which is affixed to the upper end of the outer pipe or casingC and has vertically extended guide posts 11 slidably receiving guidetubes 12 which are guided on cables R3 extending to the usual platformor vessel above or on the surface of the water. These guide tubes 12support the usual blowout preventer stack P, only the lower portion ofwhich is shown in FIG. 1, and the blowout preventer P is secured to thetop of the wellhead body 3 by suitable latch means 1... Such latch meansare well known and include releasable latch dogs 14 shiftable to innerpositions to secure the latch body 15 on the wellhead body 3 in responseto movement of an annular latch piston 16 downwardly, the latch dogs 14being releasable upon upward movement of the latch piston 16. Pistonchambers l7 and 18 are provided and are adapted to be pressurized byrespective conduits l9 and 20 to shift the piston 16 downwardly andupwardly, as desired. The blowout preventer P is connected to the latchbody 15 as by fasteners 15a.

The casing string C2 is run into the well on the drill pipe 8 until thecasing hanger 5 lands upon the casing hanger 4. At the lower end of thecasing string C2 may be a conventional casing float shoe S having theusual backflow-preventing valve 22 which prevents the flow of fluid fromthe well bore into the casing string C2, but which will permit thecirculation of fluid downwardly through the casing string C2 into thewell bore W2 and upwardly in the casing well bore annulus.

The wellhead structure described above is more or less conventional andneeds no further detailed description herein. The present inventioninvolves the cementing plug launching apparatus generally illustrated inFIG. 1 as comprising a mandrel M which is threadedly connected at 23 tothe running tool T, and extends downwardly into the upper end of thecasing string C2, the mandrel supporting at its lower end a topcementing plug TP and a bottom cementing plug BP.

As seen in FIG. 2 in greater detail, the mandrel M comprises anelongated tubular assemblage comprising a lower body section 24 and anupper body section 25 providing a central flow passage 26 extendingaxially through the mandrel. A swivel connection 27 is provided betweenthe mandrel body sections 24 and 25 to enable making up the threadedconnection 23 between the upper end of the mandrel and the running too]T while the plugs TP and HP are disposed within the casing C2, withoutrequiring that the plugs themselves rotate. The swivel connection 27 maytake any suitable form but illustratively includes an outwardlyprojecting annular flange 28 on the upper mandrel section 25 and anoutwardly extended annular flange 29 on the lower mandrel body section24 held in relatively rotatable relation by a channebshaped split ring30 which is in turn held in assembled relation with the flanges 28 and29 by a retainer ring 31. To prevent leakage through the swivelconnection, a sealing extension 32 on the mandrel body section 25 isrotatable and sealingly disposed in a bore 33 in the lower mandrel bodysection 24.

At the lower end of the mandrel M is threaded a support ring 34 to whichthe top plug TP is connected by a suitable number of circumferentiallyspaced shearpins 35. The top plug TP, as is customary, comprises atubular inner body 36 of rigid but drillable material on which is moldedan outer elastomeric body 37 having a suitable number of outwardlyextended wiper rings 38 slidably engageable with the inside wall of thecasing, and preferably provided at its upper end with an upwardlyextended sealing lip 39 also slidably engageable with the casing.Internally of the body 36 is an annular groove 40 cooperative with adart, as will be hereinafier described, to latch the dart in the topplug TP.

Means are provided for releasably connecting the bottom plug B? to thetop plug TP. In the illustrated structure, the bottom plug BP supports atubular connector 41 which is threaded into the lower end of the topplug body 36. At the lower end of the connector 41 is a ring 42 to whichthe bottom plug BP is connected by a suitable number ofcircumferentially spaced shearpins 43. In general, the bottom plug BP islike the top plug TP in that the bottom plug has a tubular inner body 44of rigid but drillable material on which is molded an outer elastomericbody having wiper rings 46 and a top ring or cup 47 slidably engageablein the casing. lntemally of the bottom plug body 44 is areduced-diameter seat 48, and at the lower end of the body is a catcher49 for a ball which, as will be hereinafter described, is adapted tocause launching of the bottom plug.

The cementing head CH, generally illustrated in FIG. 1, is adapted to beconnected to the upper end of the drill pipe 8 at the deck of the vesselor platform on or above the water. This cementing head contains a ball51 and a dart 52 which are supported in the head CH but which areselectively releasable to move downwardly through the cementing head andthrough the plug-launching mandrel M to cause the launching of the plugsBP and TP, respectively. The ball 51 is located in a bore 53 of a balldrop housing 54 which is disposed to one side of the head CH, the bore53 opening into the passage 55 extending axially through the head CH andinto the drill pipe 8. The bore 53 in which the ball 51 is retained islocated below a fluid inlet 55a. The ball 51 is retained in the bore bya slide 56 which is reciprocable in the bore by a screw actuator 57,whereby the ball may be moved from the position illustrated, into thepassage 55 so as to be dropped from the slide 56. The dart 52 isreleasably supported on a pin or slide 58 which extends into the head CHto a position beneath the dart 52 but which is retractable by a screwactuator 59. Such ball drop devices and dart-releasing devices are wellknown and require no further specific illustration or description.

The ball 51, however, is of such diameter as to readily pass downwardlythrough the pipe 8 and through the top plug TP and the connector 41, butis too large to initially pass through the seat 48 in the passagethrough the bottom plug 8?. Thus, fluid pressure may be supplied throughthe drill pipe 8 to act upon the bottom plug B? to shear the shearpins43 and launch the bottom plug. In order that fluid pressure may beeffective over substantially the entire cross-sectional area of thebottom plug, suitable radial ports 41a may be provided in the connector41 to expose the outer peripheral portion, and more particularly the cup47 of the plug BP, to fluid pressure. As will hereinafter be more fullydescribed, the ball 51 is preferably sufficiently rigid as to enable thebottom plug B? to be effectively launched by the shearing of the pins43, but when the plug is arrested in the casing, say upon contact withthe casing shoe S or other stop in the casing, the ball 51 issufficiently resilient as to be hydraulically deformed and forcedthrough the seat 48. Thereafter, the ball 51 will engage a central stopportion 49a of the catcher 49 but fluid may bypass the ball 51 throughsuitable ports 49b in the catcher 49.

The dart 52 is adapted to engage in the top plug T? to launch thelatter, and includes a rigid body 60, on an upward extension 61 of whichis mounted an elastomeric body 62 having wiper rings 63 slidablyengageable in the drill pipe 8. The body 60 supports a resilientlydeformable lock ring 64 adapted to latch the dart 52 into the top plugTl upon engagement in the annular groove 40 in the latter. When the dart52 engages the top plug TP, the pressure of fluid above the dart in themandrel M and the drill pipe 8 is relied upon to shear the shear pins 35by which the top plug Tl is releasably connected to the mandrel.

The present invention is more particularly directed to the problemsencounted in the launching of plugs in submarine cementing operationsemploying the apparatus generally described above. customarily theforces required to shear the shearpins ed to release the bottom plug andthe forces required to shear the shear pins 35 to release the top plugcan be readily calculated so that the bottom plug HP will be launchedwithout difficulty and the top plug T? will be retained on the mandrel.In this connection, in the cementing of the casing C2 in the well boreW2, it would be the usual practice to circulate a flushing fluiddownwardly through the drill pipe ll, through the mandrel M and theplugs thereon and thence downwardly through the casing C2 and into thewell bore and then upwardly through the casing well bore annulus. In anyevent, the drill pipe 8 and casing C2 would initially be full of fluid,such as drilling mud. Then a quantity of pumpable cement slurry isintroduced into the cementing head, the ball 51 being releasedsubstantially simultaneously with the flow of cement through thecementing head, the cement displacing the flushing fluid or mud from thedrill pipe as the cement travels downwardly and carrying the ball 51!downwardly. When the ball Ell seats on the seat 48 of the bottom plugBP, the shear pins d3 will be sheared, and the released bottom plug willeffectively constitute a traveling barrier between the cement slurry andthe preceding flushing fluid or mud. Generally, the weight of the cementslurry is sufficient to displace the preceding fluid, so that the cementmoves rapidly downwardly through the drill pipe, and the well goes onvacuum. Thereafter, it is the general practice to release the dart 52and to force the dart downwardly through the drill pipe with a quantityof displacing fluid which is pumped into the drill pipe and travelsrapidly and downwardly therethrough until the dart seats into the topplug TP, effectively closing d the fluid passage through the top plugTP, and resulting in a shock pressure wave which heretofore has beenknown to cause the instantaneous shearing of the shearpins which connectthe top plug to the mandrel. This is a particular problem when thepressure is applied to the entire cross-sectional area of the plug TP.The deeper the water in which the well is being completed, the moreaggravated the problem. When thetop plug T? is launched by a shockpressure wave, a pressure signal or indication is not given at the topof the drill pipe to indicate that the top plug is released. Such asignal is important to the cementing crew from the standpoint ofdetermining the subsequent volume of displacing fluid introduced intothe casing to displace the cement slurry into the casing well boreannulus.

The present invention obviates these problems by confining thedisplacing fluid to the mandrel, so that any shock pressure can act onlyon the pressure-responsive area of the dart, but thereafter the pressureacting to launch the top plug is in creased so as to produce a sensibleindication at the top of the drill pipe h caused by the drop in pressurewhen the plug TP ultimately is launched.

Accordingly, adjacent to the lower end of the mandrel body section 2dthere is mounted, as by a threaded coupling 70, a sleeve 71 definingwith the mandrel body, an annular piston chamber 72, to the upper end ofwhich fluid may be supplied through a port 73 Reciprocable in the pistonchamber 72 is an annular piston 7d having suitable side seals 75slidably engageable with the opposing cylindrical chamber walls. At itslower end, the piston 7d is in abutting engagement with the upper end ofthe rigid top plug body 36. The piston 7d and the mandrel body section24 have ports '76, 77 respectively, which are adapted to normallymaintain a balanced pressure condition across the plug assembly,particularly when the well goes on vacuum following the introduction ofcement into the drill pipe d. Stop means including a screw 7% carried bythe piston 74 and engageable with the lower end 79 of a slot 80 in thesleeve 71 are adapted to limit outward movement of the piston 74 withrespect to the chamber 72 prior to assembly with the top plug.

The ports 77 in the mandrel body section 2d are so located relative tothe seat 52a for the dart 52 that the elastomeric body, and moreparticularly the wipers 63 will close the ports 77 when the dart landsin the top plug Tl, as seen in FIG. 4 so that displacing fluid above thedart 52 cannot pass through the ports 77 and is therefore effectivelyconfined to the mandrel, and any shock pressure wave will act only onthe relatively minor cross-sectional area of the dart: 5h. The mandrelbody section 24 also has a number of ports hl which communicate with theports 73 leading to the piston; chamber 72, so that as pressure buildsup in the mandrel M and the drill pipe 8 above the top plug TP, suchpressure will be applied to the annular area of the piston 74 to producean additional downward force acting on the plug TP to shear theshearpins 35.

For purposes of some wells and some plug sizes, it will be found thatconfining the displacing fluid to the plug-launching mandrel and thesubsequent application of additional pressure to the annular piston 74is a sufficient solution to the problem of unrecognized launching of thetop plug TP. However, the present invention also provides furtherassurance that the shock pressure wave will not be effectively appliedeven to the minor cross-sectional area of the dart. This is accomplishedby providing an elongated cylindrical extension 32 on the sleeve 71, theextension 82 defining with the outer surface of the mandrel body section24 an annular surge chamber 83 which communicates through the ports hitwith the interior of the mandrel section 24 above the dart 52 when thedart seats in the top plug TP. At its upper end the extension @2 is ofreduced outside diameter so that the surge chamber volume may bemaximum. in addition, preferably, the ports hi lead upwardly at an anglefrom the interior of the mandrel section 24 into the surge chamber 83 tominimize the tendency of the cement slurry to enter the surge chamber 83and the piston chamber 72.

OPERATlON In the use of the apparatus described above, a casing stringis run into the well on a running tool T which is supported by asuitable length of the drill pipe 3, capable of extending from thewellhead apparatus H at the floor of the body of water to a vessel orplatform on or above the surface of the water. The mandrel M, with theplugs TP and BP thereon, is previously connected to the running tool,with the plugs disposed in the casing. The casing string is supported bythe casing hanger.

The casing will be filled with fluid, such as a weighted mud, and ifdesired, fluid will be circulated for a period of time to flush thecasing and the casing well bore annulus. Then, the cement slurry will beadmitted to the cementing head CH through the inlet 55a and the ballslide 56 actuated to move the ball 51 into the fluid column,approximately at the interface between the cement slurry and the mud orflushing fluid, the ball traveling downwardly in the fluid stream untilit seats on the seat M in the passage. through the bottom plug Bl. Fluidpressure on the bottom plug is admitted to the casing from the portsllla in the connector ll between the two plugs BP and TP, and causes theshearpins did to be sheared. The plug Bl then moves downwardly in thecasing, as seen in H6. 3, between the cement and the preceding fluid,acting as a barrier therebetween. When downward movement of the bottomplug BP is arrested, say by the casing shoe S, as seen in FIG. 6, theball 511 will be forced through the seat db and will be caught in thecatcher W, as cement slurry flows downwardly through the shoe S andthence upwardly in the casing well bore annulus.

As also seen in FIG. 3, as the cement moves downwardly in the casing,below the top plug Th, the mandrel M will be devoid of fluid, and thewell will be on vacuum, as the relatively heavy cement forces thepreceding fluid downwardly through the casing shoe S. At this time, theports 73 and 76 in the mandrel and in the piston 74 enable balancing ofthe pressure below the plug TP and the pressure in the annular spacebetween the mandrel M and the casing above the plug Tl, so that there isno unbalance tending to cause the release of the top plug TIP.

Next, the dart 52 is released by retraction of the dart retainer pin 58,and the dart will fall to a location below the cement head inlet 550,through which a displacing fluid of any suitable type is pumped to forcethe dart downwardly through the drill pipe, the wipers 63 wiping thedrill pipe 8 clean of residual cement.

As the displacement fluid above the dart 52 moves the dart downwardly,the dart lands in the seat 520 of the top plug TP, the latch ring 64latching into the groove in the passage through the plug TP, and thelower wiper 63 closing the mandrel port 73, so that the displacementfluid is confined to the flow passage in the mandrel. Thus, theinstantaneous shock pressure wave caused by the closure of the fluidfiowpath by the dart is limited in its application to the relativelysmall dart area. In the use of some plug sizes, the shearpins 35 can becalculated to resist such shock pressure, but for other sizes, dependingon the depth of the water, the surge chamber 83 in the mandrel M isemployed to dissipate the shock pressure wave. In other words, it iscontemplated that the confinement of fluid to the mandrel and the use ofa supplemental force to launch the top plug derived from the pressureacting on the annular piston 74 may, in some cases, be solely relied on,without a surge chamber 83, to provide a means for causing the buildupin prwsure in the drill pipe to launch the top plug, so as to provide apressure drop which is recognizable at the top of the drill pipe toindicate the release of the plug.

More particularly, as illustrated in FIG. 4, the instantaneous shockpressure wave caused by the interruption to fluid flow through the topplug Tl is dissipated in the surge chamber 83, when it is employed,since a portion of the displacing fluid above the dart can find accessto the chamber d3 through the ports 8! which are slightly spaced abovethe dart when it finally seats in the top plug. As the pressure of thedisplacing fluid increases, more of such fluid finds access to the surgechamber 83, as seen in H0. 5, until the pressure acting on the dart areaand on the annular piston 74 effects shearing of the pins 35 and theplug '1'? is forced downwardly through the casmg.

Ultimately, as seen in H6. 6, the top plug TI will land on the bottomplug Bl, downward movement of the latter having been previously arrestedby the casing shoe, as previously described, and the cement will havebeen fully displaced into the casing well bore annulus.

While the invention has been described as applicable to the cementing ofwell casings wherein both a bottom and a top plug are employed, it willbe understood that subsurface well installations of the type illustratedinvolve the cementing of certain of the larger pipe or casing strings,say on the order of or inches in diameter, wherein only a top plug isemployed. The features of the invention may also be availed of,therefore, in connection with the launching of such large top plugs.

In any event, the present invention provides plug-launching apparatuswherein the pressure-responsive area of the top plug exposed to theeffect of fluid pressure in the running string of pipe is controlled, sothat a sensible pressure rise is required to ultimately launch the plug.Such rise affords a recognizable drop in pressure when the plug isreleased. Such drop may be visually observed on a suitable pressuregauge customarily found in association with the cementing equipment.

lclaim:

1. In cementing plug apparatus for use in cementing a well pipe in awell bore, comprising: a mandrel, means for connecting said mandrel tothe well pipe with said mandrel extending longitudinally in the wellpipe, said mandrel having a passage therethrough for the flow of fluidinto said well pipe through said mandrel, cementing plug means having apassage for the flow of fluid from said mandrel passage into said wellpipe, said plug means having means cooperative with a closure in thefluid for closing said passage in said plug means, releasable meansconnecting said plug means to said mandrel and releasable by thepressure of fluid in said mandrel upon closure of said passage throughsaid plug means, the improvement wherein said mandrel has fluid pressureoperated piston means operable upon closure of said passage in said plugmeans to assist in the release of said releasable means.

2. ln cementing plug apparatus as defined in claim 1, said releasablemeans comprising shearable means connecting said plug means to saidmandrel, said piston means including an annular piston chamber in saidmandrel, an annular piston reciprocable in said chamber and extendingfrom said chamber into engagement with said plug means, and fluid inletmeans in said mandrel leading from the passage in said mandrel into saidpiston chamber.

3. In cementing plug apparatus as defined in claim 1, said mandrel alsohaving means defining a surge chamber communicating with said passage insaid mandrel for absorbing the shock pressure caused by closure of saidpassage in said plug means to prevent release of said releasable meansby such shock pressure.

4. In well-cementing apparatus as defined in claim 1, said plug meansincluding a top plug and a bottom plug, said releasable means connectingsaid top plug to said mandrel, and including additional releasable meansconnecting said bottom plug to said top plug, said bottom plug having apassage for the flow of fluid therethrough into said well pipe from saidpassage of said top plug, and means in said passage in said bottom plugcooperative with a closure in the fluid flowing through said latterpassage to close the same and effect release of said bottom plug fromsaid top plug.

5. In well-cementing apparatus as defined in claim I, said mandrelhaving port means for balancing the pressure across said plug meansuntil said passage through said plug means is closed.

6. ln well-cementing apparatus as defined in claim 1, said piston meanscomprising a sleeve disposed about said mandrel in spaced relationthereto and defining with said mandrel an annular piston chamber, fluidinlet means leading from the passage through said mandrel into saidpiston chamber, and an annular piston in said chamber and projectingtherefrom to engage said plug means.

7. In well-cementing apparatus as defined in claim I, said piston meanscomprising a sleeve disposed about said mandrel in spaced relationthereto and defining with said mandrel an annular piston chamber, fluidinlet means leading from the passage through said mandrel into saidpiston chamber, and an annular piston in said chamber and projectingtherefrom to engage said plug means, said sleeve and said piston havingcooperative stop means to prevent displacement of said piston from saidchamber prior to assembly of said plug means with said mandrel.

8. ln well-cementing apparatus as defined in claim 1, said piston meanscomprising a sleeve disposed about said mandrel in spaced relationthereto and defining with said mandrel an annular piston chamber, fluidinlet means leading from the passage through said mandrel into saidpiston chamber, and an annular piston in said chamber and projectingtherefrom to engage said plug means, said sleeve having an elongatedextension spaced from said mandrel and having sealing engagement withsaid mandrel in spaced relation to said piston means to form anelongated surge chamber communicating with said passage in said mandrelthrough said fluid inlet means to said piston chamber.

9. In cementing plug apparatus as defined in claim 1, said releasablemeans comprising shearable means connecting said plug means to saidmandrel, said piston means including an annular piston chamber in saidmandrel, an annular piston reciprocable in said chamber and extendingfrom said chamber into engagement with said plug means, and fluid inletmeans in said mandrel leading from the passage in said mandrel into saidpiston chamber, said mandrel also having means defining a surge chambercommunicating with said passage in said mandrel for absorbing the shockpressure caused by closure of said passage in said plug means to preventrelease of said releasable means by such shock pressure.

10. In well-cementing apparatus as defined in claim 9, said surgechamber communicating with said passage in said mandrel through saidfluid inlet means.

11. In cementing plug apparatus for use in cementing a well pipe in awell bore, comprising: a mandrel, means for connecting said mandrel tothe well pipe with said mandrel extending longitudinally in the wellpipe, said mandrel having a passage therethrough for the flow of fluidinto said well pipe through said mandrel, cementing plug means having apassage for the flow of fluid from said mandrel passage into said wellpipe, said plug means having means cooperative with a closure in thefluid for closing said passage in said plug means, releasable meansconnecting said plug means to said mandrel and releasable by thepressure of fluid in said mandrel upon closure of said passage throughsaid plug means, the improvement wherein said mandrel includes meansdefining a surge chamber communicating with the fluid passage throughsaid mandrel for absorbing the shock pressure wave caused by closure ofsaid passage through said plug means.

12. In cementing plug apparatus as defined in claim 11, said meansdefining a surge chamber including an elongated sleeve connected at itsopposite ends to said mandrel and defining with said mandrel anelongated annular space, and port means leading into said space fromsaid fluid passage through said mandrel.

13. In cementing plug apparatus as defined in claim 11, said mandrelhaving port means for balancing the pressure across said plug means,said port means being located to be closed by said closure when saidclosure closes the fluid passage through said plug means to confinefluid to said passage.

Q i I 0 3,1 3? UNTUCD slums PATENT 'OFIICF CERTIFICATE OF CORRECTIONPatent No. 3, 1 5 Dated November 2 97 Inventor(s) LYLE B. SCOTT It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

' 001. 2, line 56, "top plug should be bottom p1ug--.

Col. a, line 11, support should be --is supported by---. -Col. 6, line70, "ports 73 and 76" should be -ports T6 and 7?- Col. 7, line 11, "port73" should be port 77".

Signed and sealed this 2nd day of January 1973 [Sh/\L) Attcst:

ROBERT GOTTSCHALK EDWARD M. FLETCHER,JR.

Commissioner of Patents Attesting Officer

1. In cementing plug apparatus for use in cementing a well pipe in awell bore, comprising: a mandrel, means for connecting said mandrel tothe well pipe with said mandrel extending longitudinally in the wellpipe, said mandrel having a passage therethrough for the flow of fluidinto said well pipe through said mandrel, cementing plug means having apassage for the flow of fluid from said mandrel passage into said wellpipe, said plug means having means cooperative with a closure in thefluid for closing said passage in said plug means, releasable meansconnecting said plug means to said mandrel and releasable by thepressure of fluid in said mandrel upon closure of said passage throughsaid plug means, the improvement wherein said mandrel has fluid pressureoperated piston means operable upon closure of said passage in said plugmeans to assist in the release of said releasable means.
 2. In cementingplug apparatus as defined in claim 1, said releasable means comprisingshearable means connecting said plug means to said mandrel, said pistonmeans including an annular piston chamber in said mandrel, an annularpiston reciprocable in said chamber and extending from said chamber intoengagement with said plug means, and fluid inlet means in said mandrelleading from the passage in said mandrel into said piston chamber.
 3. Incementing plug apparatus as defined in claim 1, said mandrel also havingmeans defining a surge chamber communicating with said passage in saidmandrel for absorbing the shock pressure caused by closure of saidpassage in said plug means to prevent release of said releasable meanSby such shock pressure.
 4. In well-cementing apparatus as defined inclaim 1, said plug means including a top plug and a bottom plug, saidreleasable means connecting said top plug to said mandrel, and includingadditional releasable means connecting said bottom plug to said topplug, said bottom plug having a passage for the flow of fluidtherethrough into said well pipe from said passage of said top plug, andmeans in said passage in said bottom plug cooperative with a closure inthe fluid flowing through said latter passage to close the same andeffect release of said bottom plug from said top plug.
 5. Inwell-cementing apparatus as defined in claim 1, said mandrel having portmeans for balancing the pressure across said plug means until saidpassage through said plug means is closed.
 6. In well-cementingapparatus as defined in claim 1, said piston means comprising a sleevedisposed about said mandrel in spaced relation thereto and defining withsaid mandrel an annular piston chamber, fluid inlet means leading fromthe passage through said mandrel into said piston chamber, and anannular piston in said chamber and projecting therefrom to engage saidplug means.
 7. In well-cementing apparatus as defined in claim 1, saidpiston means comprising a sleeve disposed about said mandrel in spacedrelation thereto and defining with said mandrel an annular pistonchamber, fluid inlet means leading from the passage through said mandrelinto said piston chamber, and an annular piston in said chamber andprojecting therefrom to engage said plug means, said sleeve and saidpiston having cooperative stop means to prevent displacement of saidpiston from said chamber prior to assembly of said plug means with saidmandrel.
 8. In well-cementing apparatus as defined in claim 1, saidpiston means comprising a sleeve disposed about said mandrel in spacedrelation thereto and defining with said mandrel an annular pistonchamber, fluid inlet means leading from the passage through said mandrelinto said piston chamber, and an annular piston in said chamber andprojecting therefrom to engage said plug means, said sleeve having anelongated extension spaced from said mandrel and having sealingengagement with said mandrel in spaced relation to said piston means toform an elongated surge chamber communicating with said passage in saidmandrel through said fluid inlet means to said piston chamber.
 9. Incementing plug apparatus as defined in claim 1, said releasable meanscomprising shearable means connecting said plug means to said mandrel,said piston means including an annular piston chamber in said mandrel,an annular piston reciprocable in said chamber and extending from saidchamber into engagement with said plug means, and fluid inlet means insaid mandrel leading from the passage in said mandrel into said pistonchamber, said mandrel also having means defining a surge chambercommunicating with said passage in said mandrel for absorbing the shockpressure caused by closure of said passage in said plug means to preventrelease of said releasable means by such shock pressure.
 10. Inwell-cementing apparatus as defined in claim 9, said surge chambercommunicating with said passage in said mandrel through said fluid inletmeans.
 11. In cementing plug apparatus for use in cementing a well pipein a well bore, comprising: a mandrel, means for connecting said mandrelto the well pipe with said mandrel extending longitudinally in the wellpipe, said mandrel having a passage therethrough for the flow of fluidinto said well pipe through said mandrel, cementing plug means having apassage for the flow of fluid from said mandrel passage into said wellpipe, said plug means having means cooperative with a closure in thefluid for closing said passage in said plug means, releasable meansconnecting said plug means to said mandrel and releasable by thepressure of fluid in said mandrel upon closure of said passage throughsaid plug means, the improvement wherein said mandrel inCludes meansdefining a surge chamber communicating with the fluid passage throughsaid mandrel for absorbing the shock pressure wave caused by closure ofsaid passage through said plug means.
 12. In cementing plug apparatus asdefined in claim 11, said means defining a surge chamber including anelongated sleeve connected at its opposite ends to said mandrel anddefining with said mandrel an elongated annular space, and port meansleading into said space from said fluid passage through said mandrel.13. In cementing plug apparatus as defined in claim 11, said mandrelhaving port means for balancing the pressure across said plug means,said port means being located to be closed by said closure when saidclosure closes the fluid passage through said plug means to confinefluid to said passage.